This invention relates to a laser illuminating apparatus for illuminating a strip-shaped or linear area of an object, in particular sheet material.
Such illuminating apparatuses are used for example in test apparatuses for spectroscopic examination of sheetlike material, for example bank notes, ID cards and the like. Light diffusely reflected or transmitted by the illuminated strip-shaped or linear area of the object is detected by a sensor and evaluated, to detect for example the presence of errors or certain properties.
EP 0 762 174 (corresponding to DE 195 32 877) discloses an illuminating apparatus for illuminating a strip-shaped or linear area of sheet material that consists of segments of a cylindrical mirror with an elliptical base. On one of two focal lines there is a light-emitting diode array (LED array), the other focal line corresponds to the illuminated area of the sheet material. Specifically for spectroscopic examination, however, the light emitted by light-emitting diodes is unfavorable since the light is broadband. Furthermore the irradiance is low, while high irradiance in the area to be illuminated is desirable.
An older German patent application from the applicant (100 19 428.1) describes a laser illuminating apparatus for illuminating a strip-shaped or linear area of sheet material which attains improved luminance in comparison to an illuminating apparatus equipped with light-emitting diodes. Since laser applications must heed regulations for personal eye protection, the older proposal provides a fiber optic between the laser light source and the area to be illuminated for changing the laser beam cross section so that if the laser beam bundle falls on a human eye a relatively large spot with sufficiently low power density arises on the retina of the eye. However, the fiber optics involves a complex structure and furthermore leads to considerable loss of light.
DE 43 35 244 A1 discloses a so-called laser spotlight with two optical plates each in the form of a cylindrical Fresnel lens with a microlens structure. Such an assembly is not suitable for illuminating linear or strip-shaped areas, however. The large-large-surface cylindrical Fresnel lenses involve considerable losses of light in the range of 30-70% of the laser power. A similar assembly—albeit with a plurality of laser sources—is known from DE 197 09 302 A1.
To increase the detection sensitivity of a test apparatus for sheet material, high radiant power at the illuminated place is desired. High radiant powers can be obtained by illuminating apparatuses with laser light sources, for example laser diodes of laser classes 3B and 4, Nd:YAG lasers, solid-state lasers in general, in pulsed or cw mode. Wavelengths to be used range from the UV region through the visible region to the infrared region, that is, wavelengths from under 400 to about 1400 nanometers.
To guarantee personal eye protection, however, restrictive power limitations must be heeded, that is, only relatively low laser powers may be used or special saftety measures must be taken that make the overall system complicated and elaborate.